1. Field of the Invention
The present invention relates to an apparatus for reproducing information from a medium such as a disk, tape, or the like on which information was optically or magnetically recorded and, more particularly, to a digital information reproducing apparatus having a PLL circuit to produce a reference clock.
2. Related Background Art
FIG. 6 shows an example of a conventional signal reproducing circuit of a magnetic disk drive apparatus or an optical disk drive apparatus.
That is, a raw reproduction signal reproduced by a signal detector 11 comprising an optical head or a magnetic head is amplified by a preamplifier 12 and is binarized by a binarizing circuit 13 comprising a differentiating circuit, a peak detecting circuit, a comparator, and the like. A PLL (Phase Locked Loop) circuit 14 outputs a reference clock synchronized with the encoded data on the basis of the binarized signal (encoded data). The encoded data is input to a decoder 15 synchronously with the reference clock. The decoder 15 outputs the decoded data and a read clock synchronized therewith in accordance with the reference clock and the encoded data. The decoded data and the read clock become outputs of the digital information reproducing apparatus.
FIG. 7 is a diagram showing a format of the signal recorded on a recording medium in the above conventional apparatus.
On the recording medium, data is segmented and a segment unit is called a sector. One sector is constructed by a sector mark 1 indicative of the head, an address part 2 in which an address of the sector is recorded, and a data part 3 in which the user data is recorded.
FIG. 8 is a timing chart of each signal in FIG. 6 in the normal operating mode.
In the above conventional apparatus, in a case when the unrecorded sector was reproduced because of an incorrect command from a host computer to control the drive apparatus or an error of the drive apparatus or in a case when the sector of an incorrect recording state was reproduced, no reproduction signal is obtained or the reproduction signal level is low. Thus, no signal is included in the binarized encoded data and a frequency of a reference clock of the PLL circuit becomes low.
In such a case, a frequency of a read clock which is output from the decoder 15 also becomes low in response to the low reference clock frequency and a case exists wherein the number of read clocks in one sector does not reach a predetermined number, although the clocks of a number which is equal to a predetermined number of data in one sector (the number of data which can be recorded in the data part 3) should inherently be output.
FIG. 9 shows a timing chart in the case wherein such an inconvenience occurred.
Referring now to FIG. 9, since the frequency of the read clock is shifted to a low frequency, a predetermined number of read clocks are not output until the next sector mark detection signal is generated, so that its output operation is not finished.
If a predetermined number of read clocks are not generated in one sector as mentioned above, this means that the reproducing operation in one sector is not normally finished when such an operation is seen from the host computer side. Thus, a problem exists that the circuit of the host computer or the like which receives the decoded data causes an erroneous operation.
Even in a case wherein the sector in which no information is recorded was reproduced or in the case wherein the sector of an incorrect recording state was reproduced, there is also a problem that the next sector cannot be continuously reproduced.